If an odd numbered of inverters are coupled together in a loop, a ring oscillator results if the loop gain is greater than one. In contrast, if an even number inverters are coupled together in this fashion, a latch results such as a conventional SRAM cell, which is formed from a pair of cross-coupled inverters. To form a voltage-controlled oscillator (VCO), each inverter stage in a ring oscillator is configured so that its propagation delay is responsive to a control voltage. The resulting ring-oscillator-formed VCOs are important circuit building blocks in applications such as phase locked loops. Because of their common mode noise rejection and tuning properties, differential VCOs are particularly popular in such applications.
A conventional VCO 100 is illustrated in FIG. 1 having three differential inverter stages 101. As will be discussed further with regard to FIG. 2, each differential inverter stage is configured to steer a “tail current” I from a current source responsive to its differential input voltages. The propagation delay through each differential inverter stage and hence the output frequency of a differential output signal from output nodes 110 is controlled by a control voltage, Vcntl.
FIG. 2 illustrates a typical implementation for differential inverter stages 100. A differential pair of NMOS transistors Q1 and Q2 have their drains isolated from a supply voltage node Vcc by PMOS transistors M2 and M3, respectively. Each PMOS transistor M2 and M3 has its gate controlled by the control voltage signal Vcntl such that transistors M2 and M3 act as resistors in the triode mode of operation. Thus, the magnitude of the control voltage controls the resistance through transistors M2 and M3 and hence the signal delay in each inverter stage. Each transistor M2 and M3 may thus be represented by a variable resistor of resistance R determined by the control voltage. Differential input voltages Vin+ and Vin− control the gates of transistors Q1 and Q2, whose sources are tied to a current source driving the tail current I. The drains of transistors Q2 and Q1 tie to the nodes for differential output voltages Vout+ and Vout−, respectively. Because transistors Q1 and Q2 form a differential pair, virtually the entire tail current I will steer through the transistor whose gate voltage is higher than a threshold voltage multiple as compared to the remaining gate voltage. For example, if Vin+ is sufficiently higher than Vin−, the tail current steers through Q1. The output voltage Vout+ will thus be at Vcc whereas Vout− will be at Vcc−I*R, where R is the resistance of M2 and I is the tail current. These output voltages switch if Vin− is sufficiently higher than Vin+. The amplitude of the output signal is thus I*R. It can be shown that the output frequency of voltage-controlled oscillator 100 is proportional to the inverse of the propagation delay τ for each inverter stage 101. In turn, the delay is proportional to resistance R through transistors M2 and M3. Thus, the output frequency is a nonlinearly dependent on the control voltage because the control voltage controls the resistance R. It follows that the output frequency is nonlinearly dependent on the output amplitude.
This nonlinear dependence is undesirable because of the coupling of the frequency of oscillation to the amplitude of oscillation. Accordingly, there is a need in the art for a voltage-controlled oscillator having an output signal whose amplitude of oscillation is constant and independent of frequency of oscillation.